The present invention relates to high temperature, low expansion, corrosion resistant ceramic materials and, more particularly, to the use of these materials for heat exchangers in gas turbine engines. The present invention finds specific utility in the discovery that a ceramic material containing ZrO.sub.2, MgO, Al.sub.2 O.sub.3 and SiO.sub.2 as the major ingredients can be formed into a low cost, high temperature, low expansion, corrosion resistant ceramic heat exchanger suitable for use in a gas turbine engine over a wide range of high temperatures, for example as high as about 1200.degree. C.
It has previously been recognized that the continuous combustion engine (i.e., gas turbine and Sterling Cycle) offers a substantial improvement in fuel efficiency in comparison with conventional reciprocating engines. A principal problem associated with the development of the continuous cycle engine is the manufacture of a low cost heat exchanger which can also withstand the high temperature and corrosive environment present during the operation of the engine. In a typical engine application there will be one (Sterling engine) or two (gas turbine engine) wheels ranging from 20-30 inches in diameter and 3-4 inches thick rotating at about 30 rpm through a hub or ring gear mounting and drive system. During the expected operating lifetime of an automobile engine, the wheel will make over 100 million revolutions and, therefore, the heat exchanger is exposed to that many thermal shock cycles. To best withstand these thermal shock cycles, the heat exchanger must be made of a very low thermal expansion material in a form that provides a large surface area to volume ratio. Honeycomb or similar type structures where the walls are typically about 3-8 mils thick and contain from 500-1000 openings/in.sup.2 are suitable and meet these requirements.
In addition to low thermal expansion characteristics, the heat exchanger material must have excellent chemical and thermal stability. In previous work, extreme changes in the shape of the heat exchanger and, therefore, loss of its efficiency because of air leakage have occurred due to chemical changes as well as thermally induced effects in the exchanger material. The chemical changes were more severe and caused by sulfur (present in the exhaust gas) and salt (present in the incoming air).
Taking all of the above factors into account and, additionally, considering the economic problems involved have led to the selection of ceramic materials for use as the heat exchanger material. Previous work in the development of ceramic heat exchangers led to the discovery of Cordierite (Mg.sub.2 -Al.sub.4 Si.sub.5 O.sub.18) materials. These materials perform suitably in the 800.degree.-1000.degree. C. temperature range with a thermal expansion characteristic of 2.1.times.10.sup.6 /.degree.C. in that temperature range. However, there are associated with these materials recognized problems with salt corrosion and/or thermal stability. It remains as a problem in the art to develop a ceramic material which will perform satisfactorily at significantly higher tempertures (e.g., 1100.degree.-1200.degree. C.) with substantially no thermal stability or corrosion resistance problems.
As evidenced by U.S. Pat. Nos. 2,624,556; 2,937,213; 2,633,622; 2,633,623; and 3,565,645; ZrO.sub.2 -MgO-Al.sub.2 O.sub.3 -SiO.sub.2 ceramics have been used as heat exchange materials. However, none of these patents recognize that specific ZrO.sub.2 -MgO-Al.sub.2 O.sub.3 -SiO.sub.2 ceramics possess unexpectedly superior thermal expansion and corrosion resistant properties which allow them to be used in high temperature (e.g. 1200.degree. C.), high corrosive environments such as those present in gas turbine engines.